[ericgribkoff]

Eagerly initializing sslContext (#3060) can throw unnecessarily when InternalNettyChannelBuilder#setDynamicTransportParamsFactory has been used to set a transport params factory. This PR introduces DefaultNettyTransportCreationParamsFilterFactory, and moves the final sslContext member variable from NettyTransportFactory into DefaultNettyTransportCreationParamsFilterFactory. This also moves the existing nested DynamicNettyTransportParams class into DefaultNettyTransportCreationParamsFilterFactory.
Unfortunately, the git diff here is not very helpful with seeing the code movement.

I think this is a reasonable way to re-arrange things: we only use sslContext if the transportCreationParamsFilterFactory parameter to the NettyTransportFactory is null, in which case we create a new TransportCreationParamsFilterFactory, so it makes sense to move sslContext into this new class.

[carl-mastrangelo]

LGTM